Feb. 18 (UPI) -- Tennis legend Martina Navratilova wrote that it's a form of "cheating" for transgender athletes physiologically born as men to be allowed to participate in women's sports.

The 18-time Grand Slam winner wrote an op-ed piece in The Sunday Times that was posted Sunday and claimed it was "insane" that "hundreds of athletes who have changed gender by declaration and limited hormone treatment have already achieved honors as women that were beyond their capabilities as men."

In December, Navratilova became embroiled in an argument on social media with cyclist Rachel McKinnon, the first transgender woman to win a world track cycling championship, after the tennis great sent out a controversial and since-deleted tweet.

The spat led Navratilova to research the topic, and her studies reinforced her opinion that transgender women gain an unfair advantage.

"Simply reducing hormone levels -- the prescription most sports have adopted -- does not solve the problem. A man builds up muscle and bone density, as well as a greater number of oxygen-carrying red blood cells, from childhood," Navratilova wrote. "Training increases the discrepancy.

"Indeed, if a male were to change gender in such a way as to eliminate any accumulated advantage, he would have to begin hormone treatment before puberty. For me, that is unthinkable."

The 62-year-old came out in 1981 and has been a gay rights activist. Navratilova said she just wants "fairness" in sports.